System and method of determining whether to recalibrate a blood pressure monitor

ABSTRACT

A monitor for determining a patient&#39;s physiological parameter includes a calibration device that provides a calibration signal indicative of an accurate measurement of the patient&#39;s physiological parameter. The monitor also includes a processor, which receives a noninvasive signal from a noninvasive sensor positioned over a blood vessel. The processor uses the calibration signal to calibrate a relationship between the noninvasive signal and a property of the physiological parameter. The processor also determines when to recalibrate the relationship.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application No.10/052,977, filed on Jan. 17, 2002, which is a continuation of U.S.patent application No. 09/430,928, filed on Nov. 1, 1999 (the “ParentApplication”), which is a continuation-in-part of U.S. patentapplication Ser. No. 09/026,048 filed Feb. 19, 1998, now U.S. Pat. No.6,045,509, which is a continuation of U.S. patent application Ser. No.08/556,547, filed No. 22, 1995, now U.S. Pat. No. 5,810,734, whichclaims priority benefit to U.S. Provisional Application No. 60/005,519,filed October 1995, wherein U.S. Pat. No. 5,810,734 is also acontinuation-in-part of U.S. patent application Ser. No. 08/228,213,filed Apr. 15, 1994, now U.S. Pat. No. 5,590,649. The Parent Applicationand U.S. Pat. Nos. 6,045,509, 5,833,618, 5,830,131, and 5,590,649 areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an apparatus and method fornoninvasively providing a determination of a patient's physiologicalparameter and other clinically important parameters.

DESCRIPTION OF THE RELATED ART

Blood pressure is the force within the arterial system of an individualthat ensures the flow of blood and delivery of oxygen and nutrients tothe tissue. Prolonged reduction or loss of pressure severely limits theamount of tissue perfusion and could therefore result in damage to oreven death of the tissue. Although some tissues can toleratehypoperfusion for long periods of time, the brain, heart and kidneys arevery sensitive to a reduction in blood flow. Thus, during and aftersurgery, blood pressure is a frequently monitored vital sign. Bloodpressure is affected, during and after surgery, by the type of surgeryand physiological factors such as the body's reaction to the surgery.Moreover, blood pressure is manipulated and controlled, during and aftersurgery, using various medications. Often, these physiological factorsand the given medications can result in a situation of rapidly changingblood pressure requiring immediate blood pressure measurement, andcorrective action.

Because of changes in the patient's blood pressure, constant monitoringis important. The traditional method of measuring blood pressure is witha stethoscope, occlusive cuff and pressure manometer. However, thistechnique is slow, subjective in nature, requires the intervention of askilled clinician and does not provide timely readings frequentlyrequired in critical situations.

For these reasons, two methods of measuring blood pressure have beendeveloped: noninvasive, intermittent methods that use an automated cuffdevice such as an oscillometric cuff; and invasive, continuous(beat-to-beat) measurements that use a catheter.

The oscillometric cuff method typically requires 15 to 45 seconds toobtain a measurement, and should allow sufficient time for venousrecovery. Thus, at best there is typically ½to 1 minute between updatedpressure measurements. This is an inordinately long amount of time towait for an updated pressure reading when fast acting medications areadministered. Also, too frequent cuff inflations over extended periodsmay result in ecchymosis and/or nerve damage in the area underlying thecuff. The invasive method has inherent disadvantages including risk ofembolization, infection, bleeding and vessel wall damage.

To address the need for continuous, noninvasive blood pressuremeasurement, several systems were developed. One approach relies onblood pressure values in a patient's finger as indicative of thepatient's central blood pressure, as in the cases of Penaz, U.S. Pat.No. 4,869,261 and H. Shimazu, Vibration Techniques for IndirectMeasurement of Diastolic Arterial Pressure in Human Fingers, Med. andBiol. Eng. and Comp., vol. 27, no. 2, p. 130 (March 1989). Anothersystem uses two cuffs, one on each arm, to determine calibrationreadings and continuous readings respectively. Another system transformsa time sampled blood pressure waveform into the frequency domain anddetermines blood pressure based on deviations of the fundamentalfrequency. Kaspari, et al. U.S. patent application Ser. No. 08/177,448,filed Jan. 5, 1994 provides examples of these systems. An additionalclass of devices, represented by L. Djordjevich et al. WO 90/00029 (PCTApplication), uses electrical conductance to determine blood pressure.

A related area of interest was explored by perturbing the body tissue ofpatients. One class of experiments causes perturbations by inducingkinetic energy into the patient, specifically, by oscillating a bloodvessel. In the work of Seale, U.S. Pat. No. 4,646,754, an attempt isdescribed to measure blood pressure by sensing the input impedance of ablood vessel exposed to a low frequency vibration. In work by H. Hsu,U.S. Pat. No. 5,148,807, vibrations are used in a non-contact opticaltonometer. Several experiments measured the velocity of excitedperturbations in the blood and demonstrated a correlation betweenperturbation velocity and blood pressure. Such a correlation has alsobeen demonstrated between pressure and the velocity of the natural pulsewave. However, while these studies discuss the relationship betweenvelocity and pressure they do not propose a practical method ofmeasuring induced perturbations to determine blood pressure. Examples ofsuch studies are M. Landowne, Characteristics of Impact and Pulse WavePropagation in Brachial and Radial Arteries, J. Appl. Physiol., vol. 12,p. 91 (1958); J. Pruett, Measurement of Pulse-Wave Velocity Using aBeat-Sampling Technique, Annals of Biomedical Engineering, vol. 16, p.341 (1988); and M. Anliker, Dispersion and Attenuation of SmallArtificial Pressure Waves in the Canine Aorta, Circulation Research,vol. 23, p.539 (October 1968).

Known techniques for measuring propagation of pressure perturbations inarteries include Tolles, U.S. Pat. No. 3,095,872 and Salisbury, U.S.Pat. No. 3,090,377. Tolles employs two sensors to detect a perturbationwaveform and generate two sensor signals. The two sensor signals arecompared in a phase detector. The phase difference of the sensor signalsis displayed giving a signal that is capable of detecting changes inblood pressure, but which does not provide a calibrated blood pressureoutput. Salisbury similarly employs a sensor to detect a perturbationwaveform and generate a single sensor signal. The sensor signal iscompared against a reference signal. Based on the phase difference ofthe sensor signal, a universal formula is employed to determine thepatient's blood pressure. Since it has been shown, for example byLandowne, that the relationship between pressure and signal propagationvaries considerably from patient to patient, Salisbury's technique,based on a single formula, is not generally applicable.

SUMMARY OF THE INVENTION

The present invention describes an apparatus and method for measuringthe induced perturbation of a patient's body tissue to determine thepatient's blood pressure and other clinically important parameters.

An object of the present invention is to continuously determine apatient's blood pressure via a noninvasive sensor attached to thepatient.

A related object is to induce a perturbation into a patient's blood orblood vessel and to noninvasively measure the perturbation to determinethe patient's blood pressure.

A related object is to filter the noninvasive sensor signal intocomponents including a natural component, an induced component and anoise component, and to determine the patient's blood pressure from theinduced component.

A further related object is to determine a relationship between aproperty of an induced perturbation and a property of a physiologicalparameter.

A monitor for determining a patient's physiological parameter includes acalibration device configured to provide a calibration signalrepresentative of the patient's physiological parameter. An exciter ispositioned over a blood vessel of the patient for inducing a transmittedexciter waveform into the patient. A noninvasive sensor is positionedover the blood vessel, where the noninvasive sensor is configured tosense a hemoparameter and to generate a noninvasive sensor signalrepresentative of the hemoparameter containing a component of a receivedexciter waveform. In this context, a hemoparameter is defined as anyphysiological parameter related to vessel blood such as pressure, flow,volume, velocity, blood vessel wall motion, blood vessel wall positionand other related parameters. A processor is configured to determine arelationship between a property of the received exciter waveform and aproperty of the physiological parameter. The processor is connected toreceive the calibration signal and the noninvasive sensor signal, andthe processor is configured to process the calibration signal and thenoninvasive sensor signal to determine the physiological parameter. Inthe preferred embodiment, the physiological parameter measured is bloodpressure, however, the present invention can also be used to analyze andtrack other physiological parameters such as vascular wall compliance,strength of ventricular contractions, vascular resistance, fluid volume,cardiac output, myocardial contractility and other related parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages of the invention will become apparent upon readingthe following detailed description and upon reference to the drawings,in which:

FIG. 1 depicts the present invention attached to a patient;

FIG. 2 depicts an exciter attached to a patient;

FIG. 3 depicts a noninvasive sensor attached to a patient;

FIG. 4 a depicts a blood pressure waveform;

FIG. 4 b depicts a blood pressure waveform with an exciter waveformsuperimposed thereon;

FIG. 5 depicts a schematic diagram of the present invention;

FIGS. 6 a-b depict a processing flow chart according to one embodimentof the invention;

FIGS. 7 a-c are graphical illustrations of the filter procedures of thepresent invention;

FIGS. 8 a-c are graphical illustrations showing the relationshipsbetween the exciter waveform and blood pressure;

FIGS. 9 a-b depict a processing flow chart according to anotherembodiment of the invention;

FIGS. 10 a-b depict a processing flow chart according to anotherembodiment of the invention;

FIG. 11 depicts an exciter and noninvasive sensor attached to a patient;and

FIG. 12 is a flowchart showing the operation of Internal ConsistencyAnalysis according to an embodiment of the invention.

GLOSSARY

P_(D) diastolic blood pressure

P_(DO) diastolic blood pressure at calibration

P_(s) systolic blood pressure

P_(p) pulse pressure

P_(w), exciter waveform pressure

V_(d) received exciter waveform

V_(w) signal exciter waveform

V_(n) noise waveform

V_(e) exciter sensor signal (transmitted exciter waveform)

V_(p) detected pulsatile voltage

Φwexciter signal phase

Φw_(D) exciter signal phase at diastole

Vel(t) exciter signal velocity

Vel_(D) exciter signal velocity at diastole

Vel_(s) exciter signal velocity at systole

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment concentrates on the physiological parameter ofblood pressure, however, many additional physiological parameters can bemeasured with the present invention including vascular wall compliance,ventricular contractions, vascular resistance, fluid volume, cardiacoutput, myocardial contractility and other related parameters. Thoseskilled in the art will appreciate that various changes andmodifications can be made to the preferred embodiment while remainingwithin the scope of the present invention. As used herein, the termcontinuous means that the physiological parameter of interest isdetermined over a period of time, such as during the course of surgery.The implementation of portions of the invention in a digital computer isperformed by sampling various input signals and performing the describedprocedures on a set of samples. Hence, a periodic determination of thephysiological parameter of interest is within the definition of the termcontinuous.

FIG. 1 illustrates the components and configuration of the preferredembodiment. Oscillometric cuff 110 is connected to processor 100 viawire 106, and cuff 110 is responsive to processor 100 during an initialcalibration step. Oscillometric cuff operation, which is known in theart, involves an automated procedure for obtaining a blood pressuresignal. The general procedure is given for clarity but is not crucial tothe invention.

First, an occlusive cuff is pressurized around the patient's upper armto abate the blood flow. Then, as the pressure is slowly reduced, atransducer senses when the blood flow begins and this pressure isrecorded as the systolic pressure. As the pressure is further reduced,the transducer similarly detects the pressure when full blood flow isrestored and this pressure is recorded as the diastolic pressure. Thesignals representing pressure are delivered, via wire 106, to processor100 for storage. An alternative blood pressure measurement techniquesuch as manual or automated sphygmomanometry using Korotkoff sounds or“return to flow” techniques, could also be used. A manual measurementcan be provided, for example, using a keypad. Whatever measurementtechnique is used, a calibration device provides a calibration signalrepresentative of the patient's physiological parameter. In thisrespect, the calibration device is broadly defined to include automatedor manual measurements.

FIG. 1 shows an exciter 202 attached to the patient's forearm above theradial artery. The exciter 202 is a device for inducing a perturbationof the patient's body tissue, and is controlled by the processor 100 viatube 107.

FIG. 2 shows a cross section of the exciter and its components. Theexciter 202 is an inflatable bag attached to the processor via air tube107. It is fixed in place near an accessible artery 220 by holddowndevice 204 which can be a buckle, adhesive strap or other device. Thereis also an exciter sensor 203 disposed within the exciter to generate areference signal indicative of the perturbation source waveform, and todeliver the signal to the processor via wire 108. This signal is used asa reference signal by the processor (explained below). In certain cases,the exciter drive voltage or current can be used as the reference signaland the exciter sensor 203 is a simple current or voltage sensor. Insuch a case, a signal representative of the exciter current or voltageis delivered to the processor via wire 108. The exciter sensor may alsobe contained within the processor and connected to the exciter by anelectrical connection such as a wire.

As mentioned above, processor 100 is attached to the exciter via tube107. The processor 100 controls the pressure in exciter 202 with atransducer and diaphragm. A transducer is a device that transforms anelectrical signal to physical movement, and a diaphragm is a flexiblematerial attached to the transducer for amplifying the movement. Anexample of this combination is a loudspeaker. The diaphragm forms partof an airtight enclosure connected to air tube 107 and an input toinitialize the pressure. It will be clear to one skilled in the art thatthe transducer and air tube 107 and exciter 202 can be miniaturized andcombined into a single exciter element capable of acting as a vibratingair filled bag connected to the processor by an electrical drive signalalone, in the case that a source of substantially constant pressure suchas a spring is included in the exciter, or by an electrical drive signaland connection to a source of substantially constant pressure for thebag. It will be clear to one skilled in the art that the exciter mayequally well be coupled to the arm through a fluid medium such as air,or by a solid medium such as a gel.

In operation, the pressure is initially established via theinitialization input and then the pressure is varied by an electricalsignal delivered to the transducer; the diaphragm produces pressurevariations in the tube in response to the transducer movement. Theresult is that the processor, by delivering an oscillating electricalsignal to the transducer, causes oscillating exciter pressure. Theexciter responds by perturbing the patient's tissue and inducing atransmitted exciter waveform into the patient. This transmitted exciterwaveform should be considered to be a vibrational exciter waveform sinceit involves a vibrational perturbation of the tissue.

The perturbation excites the tissue 221 and blood vessel 220 below theexciter and causes the transmitted exciter waveform to radiate withinthe patient's body, at least a portion of which travels along the bloodfilled vessel. The excitation waveform can be sinusoidal, square,triangular, or of any suitable shape. Experiments conducted to determinea range of satisfactory perturbation frequencies found that the range of20-1000 Hz works well. It is anticipated that frequencies of lesser than20 Hz and greater than 1000 Hz will also work well, and it is intendedthat this specification cover all frequencies insofar as the presentinvention is novel.

FIG. 1 further shows a noninvasive sensor 210 placed at a distance fromthe exciter on the patient's wrist. The noninvasive sensor is connectedto the processor 100 via wire 109.

FIG. 3 shows a cut-away view of the noninvasive sensor 210 placed overthe same radial artery 220 as the exciter. The sensor 210 is fixed inplace near the artery 220 by holddown device 211 which can be a buckle,adhesive strap or other device. The holddown device 211 also includes abaffle 212 to reduce noise, where the baffle is a pneumatic bagpressurized to hold the sensor 210 at a constant pressure against thepatient, for example at a pressure of 10 mm Hg. Alternately, baffle 212can be any suitable device such as a spring or foam pad.

The noninvasive sensor 210 is responsive to at least one hemoparameterof the patient and generates a signal in response thereto. In thiscontext, a hemoparameter is defined as any physiological parameterrelated to vessel blood such as pressure, flow, volume, velocity, bloodvessel wall motion, blood vessel wall position and other relatedparameters. In the preferred embodiment a piezoelectric sensor is usedto sense arterial wall displacement, which is directly influenced byblood pressure. It will be clear to one skilled in the art that amicrophone or other sensor could be used as the noninvasive sensor. Thesensor may be connected to the arm by a fluid or solid coupling such asan air filled cavity or a gel.

As is shown, the sensor is positioned over the radial artery 220 and itis responsive to pressure variations therein; as the pressure increases,the piezoelectric material deforms and generates a signal correspondingto the deformation. The signal is delivered to the processor 100 viawire 109.

FIG. 1 also shows the processor 100 that has a control panel forcommunicating information with the user. A power switch 101 is forturning the unit on. A waveform output monitor 102 displays thecontinuous blood pressure waveform for medical personnel to see. Thiswaveform is scaled to the pressures determined by the processor, andoutput to the monitor. A digital display 103 informs the user of thecurrent blood pressure; there is a systolic over diastolic and meanpressure shown. A calibrate button 104 permits the user to calibrate theprocessor at any time, by pressing the button. The calibration display105 shows the user the blood pressure at the most recent calibration,and also the elapsed time since calibration. The processor maintains arecord of all transactions that occur during patient monitoringincluding calibration blood pressure, calibration times, continuousblood pressure and other parameters, and it is anticipated thatadditional information can be stored by the processor and displayed onthe control panel.

Turning to the noninvasive sensor signal, in addition to a naturalhemoparameter, the noninvasive sensor signal contains a componentindicative of the exciter waveform traveling through the patient.Although the exciter component is designed to be small in comparison tothe natural hemoparameter, it contains valuable information. Therefore,the processor is used to separate the exciter waveform from the naturalhemoparameter, and to quantify the respective components to determinethe patient's blood pressure.

FIG. 4 a shows a natural blood pressure waveform where the minimumrepresents the diastolic pressure and the maximum represents thesystolic pressure. This waveform has a mean arterial pressure (MAP) thatis a convenient reference for purposes of determining the DC offset ofthe waveform. Example pressure values are 80 mm Hg diastolic and 120 mmHg systolic respectively with a MAP DC offset of 90 mm Hg.

FIG. 4 b shows an operational illustration of the arterial waveform; anexciter waveform superimposed on a natural blood pressure waveform. Theexciter induces the exciter waveform into the arterial blood at a firstlocation and the exciter waveform becomes superimposed on the naturalwaveform. Since the exciter waveform is small compared to the patient'snatural waveform, the natural waveform dominates as shown in FIG. 4 b.As mentioned above, the noninvasive sensor signal contains informationregarding both the natural waveform and the exciter waveform. Theprocessor 100 is designed to separate the constituent components of thenoninvasive sensor signal to continuously determine the patient's bloodpressure, as is discussed below.

FIG. 5 depicts a schematic diagram of the preferred embodiment. There isan oscillometric cuff controller 121 for controlling the oscillometriccuff and determining the readings therefrom to generate a signalrepresenting the patient's blood pressure. There is an induced wavefrequency generator 131 coupled to a pressure transducer 133 thattransforms an electrical input to a pressure output. The transduceroutput is connected to the exciter 202 and controls the exciter'soscillations for inducing the exciter waveform into the patient'sarterial blood.

The output of the exciter sensor 203 is fed to a band pass filter 134.This filter 134 separates the high frequency signal responsive to thetransducer pressure and delivers the resulting signal to RMS meter 135and to lock-in amplifier 143 reference input. In the preferredembodiment, the RMS meter output is sampled at a rate of 200 samples persecond with a 14 bit resolution and delivered to computer 150. It isanticipated that the sampling rate and resolution can be varied withgood results.

The output of the noninvasive sensor is fed to a charge amplifier 140that delivers a resulting signal to a low pass filter 141 and a bandpass filter 142. These filters separate the noninvasive sensor signalinto two constituent components representing an uncalibrated naturalblood pressure wave and a received exciter waveform respectively. Thelow pass filter output is sampled at a rate of 200 samples per secondwith 14 bit resolution and delivered to computer 150, and the band passfilter output is delivered to the lock-in amplifier 143 signal input.

The lock-in amplifier 143 receives inputs from band pass filter 134 asreference and band pass filter 142 as signal, which are the excitersensor signal (transmitted exciter waveform) and noninvasive sensorexciter signal (received exciter waveform) respectively. The lock-inamplifier uses a technique known as phase sensitive detection to singleout the component of the noninvasive sensor exciter signal at a specificreference frequency and phase, which is that of the exciter sensorsignal. The amplifier 143 produces an internal, constant-amplitude sinewave that is the same frequency as the reference input and locked inphase with the reference input. This sine wave is then multiplied by thenoninvasive sensor exciter signal and low-pass filtered to yield asignal proportional to the amplitude of the noninvasive sensor signalmultiplied by the cosine of the phase difference between the noninvasiveexciter signal and the reference. This is known as the in-phase or realoutput.

The amplifier 143 also produces an internal reference sine wave that is90 degrees out-of-phase with the reference input. This sine wave ismultiplied by the received exciter signal and low-pass filtered to yielda signal proportional to the amplitude of the noninvasive sensor signalmultiplied by the sine of the phase difference between the noninvasivesensor exciter signal and the reference. This is known as quadrature orimaginary output. The amplifier 143 then provides the computer 150 withinformation regarding the real and imaginary components of the receivedexciter signal as referenced to the phase of the transmitted excitersignal. Alternately, the amplifier can provide components representingthe magnitude and phase of the received exciter signal. In the preferredembodiment, the amplifier output is sampled at a rate of 200 samples persecond with a 14 bit resolution. It is anticipated that the samplingrate and resolution can be varied with good results.

The computer 150 receives input from the oscillometric cuff controller121, RMS meter 135, low pass filter 141 and lock-in amplifier 150. Thecomputer 150 also receives input from the user interface panel 160 andis responsible for updating control panel display information. Thecomputer 150 executes procedures for further separating constituentcomponents from the noninvasive sensor signal and attenuating thenoninvasive sensor noise component as shown in FIG. 6.

While the processing system described in the embodiments involves theuse of a lock-in amplifier 143, it will be clear to those personsskilled in the art that similar results can be achieved by frequencydomain processing. For example, a Fourier transform can be performed onthe various signals to be analyzed, and processing in the frequencydomain can be further performed that is analogous to the describedprocessing by the lock-in amplifier in the time domain. The variousfiltering steps described above can be advantageously performed in thefrequency domain. Processing steps in the frequency domain areconsidered as falling within the general category of the analysis of thetransfer function between the exciter sensor waveform and thenoninvasive sensor waveform and are intended to be covered by theclaims. The variety of techniques that are used in the art for thecalculation of transfer functions are also applicable to this analysis.

-   A. Process Exciter Waveform Velocity to Determine Offset Scaling and    Exciter Waveform Amplitude to Determine Gain Scaling

FIG. 6 is a processing flowchart that represents the operation of theFIG. 5 computer 150. The operation begins at step 702 with the receiptof an initial calibration measurement; noninvasive sensor signal andexciter sensor signal. Step 704 chooses the blood pressure waveformsegment for pulse reference, which is important for continuity ofmeasurement from pulse to pulse and for consistency over periods of timebetween calibration measurements. In this embodiment, the diastolicpressure (low-point) is chosen for purposes of simplicity, but any pointof the waveform can be chosen such as the systolic pressure or meanarterial pressure (MAP). The choice of the segment will relate to the DCoffset discussed below.

Step 706 is a filter step where the noninvasive sensor (received)exciter waveform is separated into signal and noise components. Thenoise components may have many sources, one of which is a signal derivedfrom the exciter that travels to the noninvasive sensor by an alternatepath, other than that along the artery taken by the signal of interest.Examples include bones conducting the exciter waveform and the surfacetissue such as the skin conducting the exciter waveform. Additionalsources of noise result from patient movement. Examples includevoluntary patient motion as well as involuntary motion such as movementof the patient's limbs by a physician during surgery.

FIGS. 7 a-c illustrate the principles of the received exciter signalfiltering. During a natural pulse, the received exciter waveform V_(d)is represented by a collection of points that are generated in thecomplex plane by the real and imaginary outputs of the lock-in amplifier143 which is monitoring the noninvasive sensor signal. FIG. 7 arepresents the received exciter waveform V_(d) in the absence of noise.In the absence of noise, V_(d) is the same as vector V_(w)(t) which hasa magnitude and phase corresponding to the received exciter signal.During a pulse, the magnitude of V_(w)(t) remains constant, but theangle periodically oscillates from a first angle representing a lesserpressure to a second angle representing a greater pressure. Note that inthe absence of noise, the arc has a center at the origin.

FIG. 7 b represents the received exciter waveform V_(d) in the presenceof noise, which is indicated by vector V_(n). Vector V_(d) has amagnitude and phase according to the noninvasive sensor exciter waveformplus noise. As can be seen in FIGS. 7 b-c, vector V_(d)(t) defines acollection of points forming an arc having a common point V_(c)equidistant from each of the collection of points. The vector V_(w)(t)from V_(c) to the arc corresponds to the true magnitude and phase of thenoninvasive signal exciter waveform. The vector V_(n) represents noiseand, once identified, can be removed from the noninvasive sensorwaveform. The filter step removes the V_(n) noise component and passesthe V_(w)(t) signal exciter component on to step 708.

In the above discussion, it was assumed for illustrative purposes thatthe magnitude of V_(w)(t) remains constant over the time of a pulse. Insome cases the attenuation of the exciter waveform as it propagatesalong the artery is pressure dependent, and in those cases the magnitudeof V_(w)(t) can vary during the pulse in a way that is correlated topressure. Under such circumstances the shape of the figure traced out inthe complex plane by the vector V_(d) will deviate from a perfect circlesegment. A typical shape is that of a spiral with a form that can bepredicted theoretically. The functioning of this filter step under suchcircumstances is conceptually similar to that described above, exceptthat the elimination of the noise vector V_(n) must involve location ofthe origin of a spiral rather than of the center of a circle.

Step 708 determines if the pulse is valid. To do this, the processorchecks the constituent components of the noninvasive sensor signal toinsure that the components are within acceptable clinical norms of thepatient. For example, the processor can determine whether the new pulseis similar to the prior pulse, and if so, the new pulse is valid.

Step 720 processes the signal exciter waveform V_(w)(t) to determine theDC offset. For convenience the diastole is used as the offset value, butany part of the waveform can be used. The processor determines theoffset when the vector V_(w)(t) reaches its lowest phase angle (i.e.,the maximum clockwise angle of FIG. 7 a); this is the diastole phaseangle Φ_(w)(dias). A calibration diastolic measurement is stored by theprocessor at calibration as P_(DO). Also stored by the processor is arelationship denoting the relationship between the velocity of anexciter wave and blood pressure. This relationship is determined byreference to a sample of patients and is continuously updated byreference to the particular patient after each calibration measurement.FIGS. 8 a-c are graphical illustrations showing clinically determinedrelationships between the exciter waveform and blood pressure. FIG. 8 brepresents the relationship between phase and pressure at a frequency of150 Hz; other frequencies have relationships that are vertically offsetfrom the line shown. The pressure-velocity relationship represents thestorage of this graphical information either in a data table or by ananalytical equation.

Step 721 determines the predicted diastolic pressure from theinformation in Step 720. The processor continuously determines thechange in diastole from one pulse to the next by referencing theposition of the signal exciter vector V_(w)(t), at Φw(dias), withrespect to the stored pressure-velocity relationship. Moreover, thepressure-velocity relationship is continuously updated based oncalibration measurement information gained from past calibrations of thepatient.

A set of established relationships is used to develop and interpretinformation in the table and to relate the information to the sensorsignal components. First, a known relationship exists between bloodpressure and exciter waveform velocity. Also, at a given frequency manyother relationships are known: a relationship exists between velocityand wavelength, the greater the velocity the longer the wavelength; anda relationship exists between wavelength and phase, a change inwavelength will result in a proportional change in phase. Hence, arelationship exists between blood pressure and phase, and a change inblood pressure will result in a proportional change in phase. This isthe basis for the offset prediction.

With the stored calibration measurement plus the change in diastole, thenew DC offset diastolic pressure is predicted P_(D) pred). Thisprediction is made based on the diastolic pressure at calibration P_(DO)plus the quotient of the phase difference between calibration Φw_(DO)and the present time Φw(dias) and the pressure-velocity relationshipstored in processor memory as rate of change of exciter waveform phaseto pressure d(Φw_(D))/dP. $\begin{matrix}{{P_{D}({pred})} = {P_{D0} + \frac{\left( {{\Phi\quad{w({dias})}} - {\Phi\quad w_{D0}}} \right)}{{\mathbb{d}\left( {\Phi\quad w_{D}} \right)}/{\mathbb{d}P}}}} & (1)\end{matrix}$

Step 722 displays the predicted diastolic pressure.

Step 730 determines the noninvasive sensor exciter waveform phase andvelocity. This determination is made based on the comparison of thenoninvasive sensor exciter waveform with the exciter sensor waveform.

Step 731 determines the noninvasive sensor exciter waveform amplitudefrom the noninvasive sensor signal.

Step 732 determines the exciter waveform pressure P_(w) by multiplyingthe exciter sensor waveform magnitude V_(e) by the ratio of thecalibrated exciter waveform pressure P_(w)(cal) to the calibratedexciter sensor waveform magnitude V_(e)(cal). $\begin{matrix}{P_{w} = {V_{e}*\frac{P_{w}({cal})}{V_{e}({cal})}}} & (2)\end{matrix}$

In situations where a significant pressure variation can be observed inthe attenuation of the exciter waveform as it propagates from exciter todetector, an additional multiplicative pressure dependent correctionfactor must be included in equation 1.

Step 734 determines if the calibration values are still valid. Thisdetermination can be based on many factors including the time since thelast calibration, that the linearity of the pressure-velocityrelationship is outside of a reliable range, determination by medicalpersonnel that a new calibration is desired or other factors. As anexample of these factors, the preferred embodiment provides usersettable calibration times of 2, 5, 15, 30, 60 and 120 minutes, andcould easily provide more. Moreover, the curve upon which the pressureis determined is piecewise linear with some degree of overallnonlinearity. If the processor 100 determines that the data isunreliable because the linear region is exceeded, the processor willinitiate a calibration step. In one embodiment, an “InternalConsistency” trigger technique is employed to determine the validity ofthe data and to selectively trigger recalibration. This is calledInternal Consistency Analysis and is described under that heading below.Finally, the calibration can be initiated manually if the operatordesires a calibration step. A button 104 is provided on the processor100 for initiating such calibration manually.

Step 736 predicts a new pulse pressure P_(p)(pred) by multiplying theexciter waveform pressure P_(w) by the ratio of the detected pulsatilevoltage V_(p) to the detected exciter waveform magnitude V_(w).$\begin{matrix}{{P_{p}({pred})} = {P_{w}*\left( \frac{V_{p}}{V_{w}} \right)}} & (3)\end{matrix}$

This prediction uses the noninvasive sensor exciter waveform todetermine the pressure difference between the diastole and systole ofthe natural blood pressure waveform. For example, if a noninvasivesensor exciter magnitude V_(w) of 0.3 V relates to a pressure variationP_(w) of 1 mm Hg and the noninvasive sensor waveform V_(p) varies from−6 V to +6 V, then the noninvasive sensor waveform represents a pulsepressure excursion P_(p)(pred) of 40 mm Hg.

Step 760 predicts a new systolic pressure P(pred) by adding thepredicted diastolic P_(D)(pred) and pulse pressures P_(p)(pred).P _(s)(pred)=P _(D)(pred)+P _(p)(pred)  (4)

In the above example if the diastole P_(D)(pred) is 80 mm Hg (DC offset)and the pulse P_(p)(pred) represents a difference of 40 mm Hg then thenew systolic P_(s)(pred) is 120 mm Hg. Then the new systolic pressure isdisplayed.

For display purposes the values determined for P_(s)(pred) andP_(D)(pred) can be displayed numerically. Similarly, the output waveformfor display 102 can be displayed by scaling the noninvasive sensornatural blood pressure waveform prior to output using gain and offsetscaling factors so that the output waveform has amplitude, P_(p)(pred),and DC offset, P_(D)(pred), equal to those predicted in the aboveprocess. The scaled output waveform signal can also be output to otherinstruments such as monitors, computers, processors and displays to beused for display, analysis or computational input.

Step 750 is taken when step 734 determines that the prior calibration isno longer reliable as described above. A calibration step activates theoscillometric cuff 201 and determines the patient's blood pressure, asdescribed above. The processor 100 uses the calibration measurements tostore updated pressure and waveform information relating to the DCoffset, blood pressure waveform and exciter waveform. The updatedvariables include calibration pulse pressure P_(p)(cal), calibrationexciter sensor waveform magnitude V_(e)(cal), diastolic pressure P_(DO),diastolic exciter waveform phase (Φw_(DO), the rate of change of exciterwaveform phase to pressure d(Φw_(D))/dP and calibration exciter waveformpressure P_(w)(cal). $\begin{matrix}{{P_{w}({cal})} = {{P_{p}({cal})}*\left( \frac{V_{w}}{V_{p}} \right)}} & (5)\end{matrix}$

-   B. Process Exciter Waveform Velocity to Determine Offset Scaling and    Gain Scaling

FIGS. 9 a-b represent a modification to the previous embodiment. Theinitial processing steps 702, 704, 706, 708, 730 and 731 represented inthe flow chart of FIG. 9 are substantially similar to those described inthe previous embodiment depicted in FIG. 6. In step 730, exciterwaveform velocity Vel(t) and the actual phase delay of the exciterwaveform Φ(t) are related by the equation:Φ(t)=Φ₀−2πdf/Vel(t)  (6)

where frequency f and distance d between exciter and noninvasive sensorare known. The constant Φ₀ is determined in advance either analyticallyor empirically, and is dependent on the details of the geometry of theapparatus.

Measurement of Φ(t) is generally made modulo 2π, and so the measuredphase Φ_(m)(t) is related to actual phase delay by the equation:Φ_(m)(t)=Φ(t)+2nπ  (7)

where n is an integer also known as the cycle-number, typically in therange of 0-10. While correct deduction of propagation velocity requiresa correct choice of n, a correct prediction of pressure using apressure-velocity relation does not, so long as the same value of n isused in determining Φ(t) and in determining the pressure-velocityrelationship. In such a case, velocity should be considered as apseudo-velocity rather than an actual measure of exciter waveformpropagation speed.

In step 730, therefore, use of the Φ(t) equations allows determinationof the velocity, or pseudo-velocity, Vel(t) as a function of time. Instep 801, the values of velocity at the systolic and diastolic points ofthe cardiac cycle are determined as Vel_(s) and Vel_(D) These correspondto the points of minimum and maximum phase delay or to the points ofmaximum and minimum amplitude of the naturally occurring blood pressurewave detected by the noninvasive sensor. Use of the pressure-velocityrelationship stored in the processor is then made to transform thevalues of velocity at systolic and diastolic points in time to values ofpressure. In step 803 the diastolic pressure is determined using theequation:P _(D)(pred)=P _(D0)+(Vel _(D) −Vel _(D0))/(dVel/dP)  (8)

Step 804 is performed to determine the predicted systolic pressureaccording to the relationship:P _(s)(pred)=P _(D)(pred)+(Vel _(s) −Vel _(D))(dVel/dP)  (9)

In this illustration the values of P_(s) and P_(D) are used to determinethe pressure waveform. Similarly, other pairs of values, such as meanpressure and pulse pressure can also be used, and appropriatepermutations of the predicted pressure equations are anticipated by thisdescription.

In step 805 the calculated pressures are displayed as numbers, with atypical display comprising display of mean, systolic and diastolicvalues of the pressure waveform in digital form, together with theobserved pulse rate. The values of P_(D)(pred) and P_(s)(pred) are usedto determine appropriate gain and DC offset scaling parameters by whichthe naturally occurring blood pressure waveform detected by thenoninvasive sensor is scaled prior to output in step 806 as a timevarying waveform, shown as 102 in FIG. 1.

As in the embodiment depicted in FIG. 6, step 750 involves a calibrationstep initiated when step 734 determines that the prior calibration is nolonger reliable. During the performance of step 750 thepressure-velocity relationship is determined and stored in the processorin the form of a table or of an analytical relationship. During thisprocess, it may be desirable to stop the output portion of the processas shown in step 752 and display a different signal, such as a blankscreen, a dashed line display, a blinking display, a square wave, orsome other distinguishable signal of calibration such as an audibletone. This step is represented as step 808 in FIG. 9.

-   C. Process Exciter Waveform Velocity to Determine Output Blood    Pressure Waveform

In both of the previous two embodiments, values of gain P_(p)(pred) andoffset P_(D)(pred) are determined and used to scale the noninvasivesensor natural blood pressure waveform to provide a time varying outputwaveform representative of the patient's blood pressure. In thisembodiment, the natural blood pressure waveform monitored by thenoninvasive sensor is not used in the determination of the output bloodpressure waveform. As in the previous embodiment, use is made of therelationship between velocity of the exciter waveform and the bloodpressure of the patient to determine the blood pressure. Rather thanmaking such a pressure determination only at the diastolic and systolicpoints of the cardiac cycle, exciter waveform velocity is measured manytimes during a cardiac cycle (typically 50-200 times per second) and theresultant determinations of pressure are used to construct the outputtime varying blood pressure waveform. This process is described belowwith reference to FIG. 10.

In this embodiment, the natural blood pressure waveform is not scaledand the noninvasive sensor need not have significant sensitivity to thenaturally occurring physiological parameter waveform. Therefore, thereis no need to separate the data into pulse segments as in step 704 ofFIG. 6. This feature greatly simplifies the computational task. Anadditional advantage of this technique is that all of the informationused in the analysis process is encoded in the exciter waveform, whichis typically at a high frequency compared with that of both the naturalblood pressure waveform and that of any artifact signals introduced bypatient motion or respiration. Since all of these lower frequencysignals can be removed by electronic filtering, this technique isextremely immune to motion induced artifact and similar sources ofinterference that might otherwise introduce errors into the measurement.

With the exception of this step, the initial processing steps 702, 706,731 and 730 are substantially similar to those of previously describedembodiments. The amplitude and phase of the exciter waveform determinedin step 731 are continuous functions of time. The exciter waveform phaseis converted to exciter waveform velocity as described previously, whichis also a continuous function of time.

Using a relationship between pressure and velocity, determined during orsubsequent to the initial calibration and periodically redetermined, thetime dependent velocity function Vel(t) is readily transformed to a timedependent pressure function P(t). This transformation is represented bystep 802. In a typical case, the pressure velocity relationship might beas follows:Vel(t)=a+bP(t)  (10)

where the constants a and b were determined during step 750. In thatcase the velocity equation (10) can be used to perform thetransformation of step 802.

Following a variety of checking steps, described below, that ensure thetransformation used in 802 was correct, the minimum and maximum pointsof P(t) are determined for each cardiac cycle and displayed asP_(D)(pred) and P_(s)(pred) in step 805. Then, in step 806, the entiretime dependent waveform is displayed as waveform 102.

-   D. Determination of the Pressure-Velocity Relationship

In each of the embodiments described thus far, an important stepinvolves the conversion of a measured phase to a deduced exciterwaveform velocity, and conversion of that value to a pressure. In thecase of the flow chart of FIG. 6, this process is integral to thecalculation of the DC offset pressure P_(DO). In the case of theembodiment described in FIG. 9, this process is integral todetermination of P_(s) and P_(D). In the case of the embodimentdescribed in FIG. 10, the process is integral to the determination ofpressure at each point in time for which an output value is to bedisplayed as part of a “continuous” pressure waveform display.

The relationship between pressure and velocity is dependent on manyfactors including the elastic properties of the artery along which theexciter waveform travels. This relationship varies considerably betweenpatients, and must therefore be determined on a patient by patientbasis, although a starting relationship derived from a pool of patientscan be used. This determination occurs during step 750 in each of theembodiments described in FIGS. 6, 9, and 10, and the relationship isstored in the processor in either a tabular form, or as an analyticalrelationship. In step 734 in FIGS. 6, 9 and 10, a variety of parametersare examined to determine whether the system calibration continues to beacceptable. As part of that process, it is determined whether theexisting pressure-velocity relationship continues to be valid. If not, arecalibration can be initiated. Such a determination can be performedusing techniques described earlier or those using multiple perturbationsdescribed below. One aspect of the multiple perturbation determinationis presented under the heading Internal Consistency Analysis.

-   E. Multiple Perturbations

For each of the different embodiments described hereto, an additionalembodiment is described using multiple perturbation waveforms. All thefeatures and advantages of the prior embodiments are applicable to theseembodiments.

In the case of each of the previously described embodiments anembodiment is described in which the apparatus further induces a secondexciter waveform into the arterial blood. An example second exciterwaveform is one that has a frequency different from that of the firstexciter waveform. It is noted that although the discussion of the secondembodiment concentrates on a second exciter wave, any number of two ormore exciter waves can be used to determine the perturbation velocitymeasurement.

In operation, processor 100 generates two exciter waveforms andcommunicates the waveforms to the exciter 202 via air tube 107. Theexciter 202 responds by inducing both exciter waveforms into thepatient. Noninvasive sensor 210 generates a signal responsive to ahemoparameter and transmits the signal to the processor 100 via wire109. As described previously, the exciter may also be an element, suchas a loudspeaker, physically attached to the body, in which case theexciter waveforms are sent from the processor to the exciter by means ofan electrical connection or wire.

The processor filters the noninvasive sensor signal into components ofthe natural waveform, a first exciter waveform, a second exciterwaveform and noise. The processor determines the phase relationship ofthe first exciter waveform to a first reference input and determined thephase relationship of the second exciter waveform to a second referenceinput.

Once the processor has determined the phase of the exciter waveforms,the processor then generates a plurality of points, the slope of whichrelates to the velocity of the exciter waveform. This is shown in FIG. 8c, where the slope of the line is 2πd/Vel, and where d is distance andVel is velocity. Since the distance is fixed and the slope is related toblood pressure, and since the slope changes based on changes in bloodpressure, the velocity of the exciter waveform is determined.

The technique described above yields a measurement of the groupvelocity. In contrast, the techniques described in previous embodimentsresult in the measurement: of a phase velocity or of a pseudo-phasevelocity in the case that the value of n of the phase equation (7) cannot be uniquely determined. In a dispersive system these values need notalways agree. However, phase, group and pseudo-velocity aremonotonically varying functions of pressure. Thus, a measurement of anyone of the three is a basis for a pressure prediction, so long as theappropriate pressure-velocity relationship is used. These parameters canbe used for the Internal Consistency Analysis trigger described belowunder the same heading.

An additional benefit of the use of multiple frequency perturbations isthat it allows the unique determination of the value of n in the phasemeasurement equation described above. This allows the use of actualphase velocity, rather than of the pseudo-velocity described earlier inthe multi-perturbation analogues of the embodiments depicted in FIGS. 6,9 and 10.

Once the velocity is determined, a prediction of blood pressure is madeaccording to FIG. 8 a, showing the relationship of velocity to pressure.Thus, it is possible to determine the blood pressure with few, or zero,calibrations.

Another embodiment is depicted in FIG. 11 showing a cross section of anexciter 202 and noninvasive sensor 210 at the same position above theblood vessel 220. The proximate location of the exciter and the sensorpermits measurement of the blood vessel's response to the perturbations.In this embodiment, the noninvasive sensor is responsive to ahemoparameter such as blood flow or blood volume. These parameters canbe measured with a sensor such as a photoplethysmograph. Detectedchanges in the blood vessel due to the natural pulsatile pressure arecalibrated using external exciter pressure oscillations and comparedagainst the sensor signal by the processor.

-   F. Internal Consistency Analysis

Internal Consistency Analysis (ICA) relates to the use of informationmeasured at multiple frequencies to monitor the state of a system, asthat state pertains to the behavior of other parameters of the system.The monitoring of the variability or constancy of the state of thesystem can be used to evaluate the current validity of the expectedbehavior of the other parameters. The system may be physiological orotherwise. The multiple frequency information may be in the form ofdiscrete or continuous frequency content. The information may beutilized simply, such as in the dependence of a given parameter onfrequency, or in more complex ways, such as in the dependence of twonon-frequency parameters on each other while taking advantage of therange of parameter space spanned by the data from a range of frequency.

In the embodiment described here, ICA is used in a monitor fordetermining a physiological parameter of a patient. In such a monitor,ICA is used to determine when the state of the patient has changed insuch a way as to make desirable either a recalibration of the monitoringsystem, or a correction to the measurement to compensate said change ofstate of the patient. The use of ICA for such purpose is described indetail for the case of monitoring of the blood pressure of a patient.However, the principles of ICA work equally well in monitors of otherphysiological parameters by the techniques described in this invention.

The physiological state of an artery over the time scale of a bloodpressure monitoring period is determined primarily by the level ofactivity of the smooth muscle component of the artery wall. The level ofsmooth muscle activity affects both the elastic and viscous propertiesof the artery wall. The elastic properties of the wall are the maindeterminants (although viscous properties are involved as well) of therelationship between velocity and pressure (the V-P relationship—seeequation 10). This is equally true of relationships betweenpseudo-velocity or phase and blood pressure. The viscous properties ofthe wall have a significant influence on a number of observablequantities or relationships, such as the dependence of the propagationvelocity on frequency, the propagation attenuation, and others. Theseobservable quantities or relationships can be used to monitor thephysiological state of the artery, and hence to monitor the validity ofa previous V-P relationship calibration. These observable quantities orrelationships may also be used to continuously track, calculate thechanges in, and deduce the current V-P relationship.

A number of observable quantities or relationships have been studied ina series of clinical investigations that we have caused to be carriedout. Several were found to be well-correlated with the V-P relationship.Two observable quantities called dispersion (D) and attenuation (A) wereidentified as of particular importance although several other quantitieswere also viable candidates for use in ICA.

The D quantity, as implemented, was calculated as follows. Firstcalculate the propagation velocities for the frequencies of 300, 400,500, 600, and 700 Hz as a continuous function of time as the bloodpressure pulses. Take the mean of these velocities over a pulse, toproduce the velocity at Mean Arterial Pressure (MAP). Take a linearregression of the velocity at MAP vs. frequency using the valid datafrom all five frequencies. The slope of this line is the dispersion (D).This is an example of a utilization of the multiple frequencyinformation, because the calculated quantity is a relationship betweenanother parameter and frequency.

The attenuation (A) quantity, as implemented, was calculated as follows.First calculate the propagation phase delays for the frequencies of 300,400, 500, 600, and 700 Hz as a continuous function of time as the bloodpressure pulses. Also calculate the propagated amplitude of the exciterwaveform (relative to the injected initial amplitude) for thefrequencies of 300, 400, 500, 600, and 700 Hz. Calculate the quantityln(GV²/sin(kL/2)) for each frequency, where G is the relative propagatedamplitude, v is the propagation velocity, L is the exciter length, and kis the propagation wave vector. Take a linear regression ofln(Gv²/sin(kL/2)) vs. propagation phase using the valid data from allfive frequencies. The intercept of this line at a phase of −4 radians isthe attenuation (A). This is an example of a more complex utilization ofthe multiple frequency information, because the calculated quantity is arelationship between two parameters, neither of which representsfrequency directly, but the relationship is calculated over a range ofparameter space spanned by data from a range of frequency.

There are a number of other observable quantities or relationships andminor modifications on the dispersion (D) and attenuation (A) quantitieswhich may be useful. For example, the described dispersion (D) iscalculated at MAP, but it could also be calculated at a differentconstant blood pressure. Also the described attenuation (A) utilizes anintercept of the regression of ln(Gv²/sin(kL/2)) vs. propagation phase,but the slope of this line is another useful quantity.

FIG. 12 is an example flow chart of an embodiment using the aboveobservable quantities or relationships to implement a recalibrationtriggering technique. The observable quantities or relationships thatindicate the state of the system and that can be measured betweencalibrations, such as the dispersion (D) and attenuation (A) discussedabove, are denoted by the term “trigger parameters” (TP). The parametersdescribing the behavior of the system that depend on what state thesystem is in (such as the relationship between velocity and pressuredescribed above), which we are not generally monitoring betweencalibrations but rather utilizing in the signal processing to produce acalculated output (such as the monitored blood pressure), are denoted bythe term “state parameters” SP.

Note that in general each of the terms “trigger parameter” and “stateparameter” may indicate parameters of one or more dimensions. Forexample the dispersion parameter (D) described above, which is a triggerparameter (TP), is one numerical scalar value and is, hence, onedimensional. The combination of D and A is considered a two dimensionaltrigger parameter. The V-P relationship described above as the primarystate parameter is two dimensional if the relationship is modeled as aline, but in general could take on higher or lower dimension. In thediscussion that follows the term trigger parameter TP is understood toencompass both the possibility of a single dimensional andmulti-dimensional TP. Likewise, the term state parameter SP isunderstood to encompass both the possibility of a single-dimensional andmultidimensional SP. The same concepts can be applied to morecomplicated quantities using more sophisticated vector, matrix, ortensor mathematics if necessary.

To utilize a TP to deduce a change in SP, we need to know therelationship between TP and SP, for example, how has SP changed if wemeasure a change in TP. This relationship is denoted with the functionF, so that SP2=F(SPI) TPI, TP2). That is to say SP at time 2 is afunction of SP at time 1, TP at time 1, and TP at time 2. The function Fcan generally be quite complicated, or it can be as simple as a linearscaling from the change in TP to the change in SP, for example, (SP2−SP1)=X*(TP2 TP 1) where here X is a constant.

The specification of the function F shown as steps 904 and 918 in FIG.12 can be accomplished in a number of ways. The simplest way to producean initial F (as in step 904) is to use a pool of previously collectedpatient data, containing information on both TP and SP over a range oftime and conditions, to develop an approximation of the function. Ifthis initial approximation is found to produce acceptable results intesting, there is no need for step 918. If testing shows that theinitial approximation of F is inadequate, F can be updated for eachpatient (as in step 918) using the information obtained by thecalibration process. It may be sufficient to rely on calibrations thatoccur for other reasons to accomplish this updating process, or thedevice could undergo a series of planned calibrations specifically forthis purpose.

At the time of calibration, both initial and subsequent, shown as step906 in FIG. 12 SP1 is determined from the combination of informationfrom the noninvasive sensor signal and from the calibration device, andTP 1 Is determined from the noninvasive sensor signal.

After calibration the device goes onto continuously obtain thenoninvasive sensor signal at time 2, shown as step 908 in FIG. 12. Inthe notation of FIG. 12, time 1 is the time of the last calibration, andtime 2 is the subsequent time of continuous, monitoring operation.

Given the continuous noninvasive sensor signal, and the SP 1 obtained bythe previous calibration, the device can then produce an output bloodpressure, as is shown in step 916.

Given the continuous noninvasive sensor signal, the device can calculateTP2 as is shown in step 910.

Given TP2, SP1 and TP1 combined with the function F, the device canestimate the current SP2 as is shown in step 912.

At this point there is an optional variation on the recalibrationtechnique. If it is determined in testing that the estimate of SP2produced in step 912 is sufficiently accurate, the continuousnoninvasive sensor signal and SP2 can be used to produce an output bloodpressure, as is shown in step 920. In this variation step 920 replacesstep 916, which is not performed.

In step 914 the current estimate of SP2 is compared to the SP1 which wascalculated at the last calibration. If the change in SP exceeds apredetermined recalibration threshold, the device is instructed toperform a new calibration in step 906. If the change in SP does notexceed the predetermined recalibration threshold, the device loops backto step 908 to continue acquiring the noninvasive sensor signal andproducing blood pressure output. Specification of the recalibrationthreshold is based on the amount of change in SP that is tolerablebefore unacceptably inaccurate results are produced. Step 918, shown indotted lines, is optional. Step 920, shown in dotted lines, is anoptional replacement for step 916.

Note that there are many possible ways to implement a recalibrationtriggering technique, and the simple example shown in FIG. 12 is but onepossible methodology. A few possible modifications to the techniqueinclude: (a) to decide when to recalibrate based on changes in TP ratherthan on the changes in SP that are calculated from the changes in TP;(b) to recalibrate based on TP or SP reaching or passing through certainpredetermined values rather than changes since the last calibration; or(c) combinations of these and other variations.

The specific detailed embodiment described above is one of severalviable trigger techniques using ICA. Other variables that may be usedinstead of, or in conjunction with, the variables D and or A, includebut are not limited to the following: (a) the slope or intercept of arelationship between amplitude (G) and phase of the signal exciterwaveform that is expressed in a different functional form than theexample used above of ln(GV²/sin(kL/2)) or various comparative measuresof such slope or intercept derived using differing frequencycombinations; (b) the difference between the phase velocity or groupvelocity of the signal exciter waveform (or the intercept of the line ofbest fit to data on a graph of omega versus k, where omega is equal tofrequency times two pi); (c) various pulse shape characteristics ofeither the physiological parameter waveform or the exciter signalvelocity waveform such as the ratio of the power in components offrequency greater than 5 Hz to the total power in components of greaterthan 0.2 Hz (or differences between these two waveforms or betweenwaveforms from different exciter frequencies or different exciters ordetectors); (d) heart rate, pulse pressure, pressure, otherphysiological parameters and ratios thereof; (e) various permutations ofthe above techniques using metrics derived from more than one detectorand or exciter and examining the internal consistency thereof.

The general philosophy of ICA is to use multiple parameters to examinethe internal consistency of information derived in different ways fromseveral sources. A further embodiment of ICA involves the use ofmultiple exciter waveforms as described in section E. Each of thesefrequencies of perturbation can be used to derive an independentdetermination of a physiological parameter using the techniquesdescribed in this invention. So long as these independent determinationsare in reasonable agreement, the system can be allowed to continue tomonitor the physiological parameter. If the independent determinationsdiffer by more than some threshold, then a recalibration can betriggered.

In a further embodiment of ICA, the velocity of propagation of thecardiac pulse between two locations on the body is measured usingtechniques such as are described in U.S. Pat. No. 5,785,659,incorporated herein by reference. This velocity is also related to bloodpressure and can be used as an independent predictor of blood pressureonce it has been calibrated appropriately. Various changes in thisvelocity, or differences in the pressure predicted using it and thepressure predicted using the techniques of this invention can be used totrigger a recalibration.

-   G. Variations on the Disclosed Embodiments

Additional embodiments include an embodiment in which two or moredetectors are positioned along the artery at different distances from asingle exciter, and an embodiment in which two or more exciters arepositioned along the artery at different. distances from one or moredetectors. In each of these embodiments, the information. obtained fromeach exciter detector pair can be analyzed independently. The multiplyredundant measurements of pressure that result can be combined toprovide a single pressure determination that may be both more accurateand more immune from noise, motion artifact and other potential errorsources. Similar redundancy can be achieved in the embodiments that usemultiple exciter waveforms by analyzing the results at each frequencyindependently and combining the results to provide enhanced robustness.

In addition, any combination of more than two elements (e.g. twoexciters and one detector, two detectors and one exciter, one exciterand three detectors) allows the value of n in the phase equation (7) tobe uniquely determined so long as the spacing of two of the elements issufficiently small to be less than a wavelength of the propagatingperturbation. Since the possible range of perturbation wavelengths at agiven-pressure can be determined from a pool of patients, selection ofthe appropriate spacing is straightforward and can be incorporated intothe geometrical design of the device.

-   H. Conclusion

A close relationship between physiological parameters and hemoparameterssupplies valuable information used in the present invention. Theperturbation of body tissue and sensing the perturbation also suppliesvaluable information used in the present invention. Although thepreferred embodiment concentrates on blood pressure, the presentinvention can also be used to analyze and track other physiologicalparameters such as vascular wall compliance, changes in the strength ofventricular contractions, changes in vascular resistance, changes influid volume, changes in cardiac output, myocardial contractility andother related parameters.

Calibration signals for the present invention can be obtained from avariety of sources including a catheter, manual determination, or othersimilar method.

The DC offset for the physiological parameter waveform can be obtainedin a variety of ways for use with the present invention.

The exciter of the preferred embodiment uses air, but any suitable fluidor solid can be used. Moreover, various exciter techniques can be usedfor inducing an exciter waveform into the patient such as an acousticexciter, an electromagnetic exciter and an electromechanical exciter(e.g. piezoelectric device).

Various noninvasive sensors have been developed for sensinghemoparameters. These sensor types include piezoelectric,piezoresistive, microphones, impedance plethysmograph,photoplethysmograph, various types of strain gages, air cuffs,tonometry, conductivity, resistivity and other devices. The presentinvention can use any sensor that provides a waveform related to thehemoparameter of interest.

Having disclosed exemplary embodiments and the best mode, modificationsand variations may be made to the disclosed embodiments while remainingwithin the subject and spirit of the invention as defined by thefollowing claims.

1-23. (canceled)
 24. A system for monitoring a physiological parameter,comprising: an inflatable blood pressure cuff configured to provideblood pressure calibration data for a patient; an exciter configured toinduce a transmitted exciter waveform into the patient; aphotoplethysmograph configured to sense a first hemoparameter comprisingblood oxygenation; a noninvasive sensor configured to sense data relatedto a second hemoparameter; and a processor configured to compute aphysiological parameter using said calibraton data and said data relatedto said second hemoparameter, said processor configured to activate saidinflatable blood pressure cuff and obtain said blood pressurecalibration data when a value of said first hemoparameter indicates thatrecalibration is needed to obtain a desired level of accuracy.
 25. Thesystem of claim 24, wherein said physiological parameter comprises bloodpressure.
 26. The system of claim 24, wherein said exciter induces awave propagating along an artery of the patient.
 27. The system of claim24, wherein method of claim 1, said second non-invasive sensor islocated at a first position on an extremity of said patient and saidexciter is located at a second position on said extremity.
 28. Thesystem of claim 24, wherein recalibration is performed when said firsthemoparameter exceeds a threshold value.
 29. The system of claim 24,wherein said processor is further configured to activate said bloodpressure cuff to obtain said calibration data at specified intervals.30. A system for monitoring a physiological parameter, comprising: aninflatable blood pressure cuff configured to provide blood pressurecalibration data for a patient; an exciter configured to induce atransmitted exciter waveform into the patient; one or more sensorsconfigured to produce sensor data related to one or more hemoparameters;and a processor configured to compute blood oxygenation from said sensordata, said processor further configured to compute a physiologicalparameter using said calibration data and said sensor data, saidprocessor configured to activate said inflatable blood pressure cuff andobtain said blood pressure calibration data when a value of said bloodoxygenation indicates that recalibration is needed to improvecomputation of said physiological parameter.
 31. The system of claim 30,wherein said physiological parameter comprises blood pressure.
 32. Thesystem of claim 30, wherein recalibration is performed when said firsthemoparameter exceeds a threshold value.
 33. The system of claim 30,wherein said processor is further configured to activate said bloodpressure cuff to obtain said calibration data at specified intervals.34. The system of claim 30, wherein said processor is further configuredto activate said blood pressure cuff to obtain said calibration dataduring system initialization.
 35. A system for monitoring aphysiological parameter, comprising: an inflatable blood pressure cuffconfigured to provide blood pressure calibration data; an exciterconfigured to induce a transmitted exciter waveform into the patient; afirst noninvasive sensor configured to sense a first hemoparametercomprising blood oxygenation; a second noninvasive sensor configured tosense data related to a second hemoparameter; and a processor configuredto compute a physiological parameter using said calibration data andsaid data related to a second hemoparameter, said processor configuredto activate said inflatable blood pressure cuff and obtain said bloodpressure calibration data when a specified change occurs in said firsthemoparameter.